EA2007/EA5017: Soil Properties and Processes - Essay Writing Assessment Answer

February 20, 2018
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Soil Properties and Processes

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Factors determining the availability of phosphorus to plant roots in soil

Abstract

Phosphorus is the second most important element needed by the plants to carry out many of their physiological and biochemical processes. It is an essential part of the structure of DNA, RNA and cellular membranes. It also stores and transports the energy released during the process of photosynthesis. It also regulates important reactions by altering the activity of crucial enzymes with of kinases. Even though the amount of phosphorus present in the soil is adequate compared to the requirements of the plants, it was observed that not all the phosphorus was available to the plant for use. There many factors which influence this process. Here in this essay, we have tried to discuss some of these factors which make phosphorus available to the plants.

Introduction

After nitrogen, phosphorus (P) is the next most limited nutrient for plants (Hinsinger 2001). It plays an important role in many physiological and biochemical processes. Its basic function is to make the energy available from photosynthesis for growth and reproductive processes. It is also a part of the structure of nucleic acids- both DNA and RNA, and is also constituent of the cell membrane in plants. Sufficient levels of phosphorus help promote root growth, winter hardiness and decreases maturity time in plants. It has the ability to modify the activity of important enzymes and help in cell signaling. Phosphorus generally concentrates in the parts of plants which actively grow (Theodorou and Plaxton 1993). Phosphorus in soil in present is a variety of organic and inorganic forms as shown in figure 1 (Vance, Pierzynski et al. 1994). Organic phosphorus is generally present in humus and other organic material, which gets released by the process of mineralization upon action of soil organisms. Inorganic phosphorus is taken up by plants in the form of H2PO4- and HPO42-.

Soil Properties and Processes

Availability of phosphorus in soil

Unavailability of phosphorus can limit the growth of plants and hence is a major constraint for their growth. The phosphate present in soil becomes immediately available to the plants however the amount present is usually too low in comparison to what is needed by the plants. Though highly fertilized soil has higher concentration of P ions, the concentration in general lies in micromoles ranging from 0.1 to 10 ?M. These values are very low compared to the optimal requirement of the plants which can reach up to tens of ?M for highly demanding crops (Hinsinger 2001). In many conditions, even when the soil P concentration upon estimation seems adequate plants suffer from deficiency of P. It is attributed to the unavailability of the P as some portion of the estimated P is in solid phase, as a chemical part of other soil minerals or fixed into organic forms. This solid phase P thus becomes inaccessible to the plants. So the accessibility of phosphorus preset in the soil depends upon its chemical nature; only the inorganic phosphate in solution can be used up by the plants for their needs. The physical characteristics of the soil also influence the P availability to plants as these factors determine the rate at which phosphate gets transport to the roots. Additionally, the features of plants also control the acquisition of P from the soil.

So, chemical and physical nature of the soil and the characteristics of the plant together govern the availability of phosphorus to the plants.

Phosphorus chemical behaviors in soil

As already mentioned the availability of phosphorus gets affected by the chemical properties in the soil. The pH of soil greatly affects the extend up to which P will be available for the plant to uptake (Gahoonia and Nielsen 1992). Phosphorus uptake behaves differently in different soil type. In alkaline soil the uptake of P increases with a decrease in the pH and it is said to be because of the dissolution of P minerals. On the other hand, increasing the pH in acidic soil leads to more uptake of P. It is believed to happen because of the desorbing P from metal oxides (Gahoonia and Nielsen 1992). Figure 2 shows the availability of P to plants as a function of pH.

Soil Properties and Processes

The presence of P in soil is of utmost important for plant productivity. The P recovered from the soil in the growing season is generally very low as most of it becomes immobile and thus in accessible to the plant due to adsorption, precipitation and conversion into organic form (Holford 1997; Mengel and Kirkby 1982). Phosphorus chemically reacts with positively charged oxides of iron, aluminium, and calcium ions thereby forming some insoluble forms in the soil, called the non-labile phosphorus (Mengel and Kirkby 1982). Speciation, precipitation-dissolution and adsorption-desorption of P are the three important methods controlling the movement of P ions in soils determining its bioavailability to plants in inorganic form. A part from orthophosphate ions, different -, + and uncharged species of P are also present in the soil solution, however abundance of its existence depends on the pH and the amount of metal cations such as Ca, Fe and Al and organic and inorganic ligands. Most of the forms that P takes bear negative charge therefore mostly positively charged P sorbents are found in the soil. These species mostly constitute hydroxyl (Fe and Al oxides), carboxyl (organic matter) or silanol (clays) groups. The oxides are commonly found in ferralsols from tropical regions and the soils with alkaline pH such as calcareous soils (Hinsin??_ger 2001). The equilibrium maintained by the process of precipitation and dissolution, responsible for the solubility of P in the soil, is thus dependent on pH of the soil, concentration of P and also the concentration of the mentioned metal cations. The unavailable P, usually high in amount, is available outside the rhizosphere where roots can’t reach (Schachtman, Reid et al. 1998). Therefore the availability of P becomes dependent on how quickly the pool of P can be replenished (Ryan, Delhaize et al. 2001). Figure 3 below depicts the various factors and processes related to P availability in the rhizosphere (Khorassani 2008).

Soil Properties and Processes

As shown in figure 2, when pH is greater than or equal to 7 (neutral or alkaline soil), P precipitate to form Ca phosphates, which mainly include dicalcium or octacalcium phosphates, hydroxyapatite and insoluble apatites. But in acidic soil (pH<7) precipitates of Fe and Al phosphates are generally found. These include compounds like strengite, vivianite, variscite and various minerals of the plumbogummite group (Hinsinger 2001).

The chemical processes regulating the concentration of free phosphorus in soil are adsorption and desorption involving various soil constituents. P uptake by plants is related to sorption and depends upon the amount that can be extracted from the compounds of iron and aluminium in both acidic and calcareous soils. Phosphate released from the oxides of metal is a crucial source of P for the plants as reported in the literature. P is desorbed from the soil with the help of ligand exchange process in which the decreasing concentration of P ions in the soil is compensated with an increase in the concentration of some other competing anions shifting the adsorption– desorption equilibrium towards enhanced desorption (Hinsinger 2001; Khorassani 2008).

Transportation of phosphorus in soil

The nutrients move from the soil to the target roots because of the interaction between plant roots and some physical factors governing the particles of soil. As P is strongly bound to various constituents in the soil, its movement within the soil becomes too low and only a very small portion of inorganic P present in the form of phosphate ions reaches the plants. Hence, physical factors in soil play a crucial role in transportation of P towards the plant roots, especially when the concentration P in soil in very less. Transportation thus becomes an important factor governing the availability of P to plant roots. Figure 4 shows the distance from the root up to which plants can uptake P.

Soil Properties and Processes

Mass flow and diffusion are the two processes which give an estimate of the total flux of P reaching the roots from the soil. About 95% of the P absorbed by the plants reaches the site of exchange by the process of diffusion. The laws governing the process of diffusion were established by Fick in 1855. According to his law, the flux by diffusion (FD) is directly proportional to the concentration gradient of the ions (?C/?x ). Using the effective diffusion coefficient, De, Rate of diffusion can be represented as the following equation,

FD=-De?C?x

where C is the concentration of ions participating in the process of diffusion per unit volume of soil. This accounts from all the P ions- present in soluble form as well as in bound state which may or may not get released. The diffusion coefficient (De) is given by (Nye 1966),

??_

De=DLf1b

where, DL refers to the diffusion coefficient in water, is the volumetric soil water content, f is impedance factor, b is soil buffer power. It takes into consideration that only a fraction of the total ions present in the soil will be participating the process of diffusion (Jungk and Claassen 1997). , expressed as volume of water per volume of soil also affects the supply of nutrients to the plants. It was shown in an experiment that increasing to the range found in Luvisol, the effective P diffusion coefficient was also increased up to 25 folds (Bhadoria, Kaselowsky et al. 1991).

While diffusion, the ions actually have to travel a distance much longer than the straight distance between the start and the end points. This is defined by the above mentioned impedance factor, which is also known as tortuosity factor and is given by

f=LLe2

where L is the shortest distance between the two points marking the start and end of diffusion, and Le is the actual distance covered by the ions starting and ending at the same points. The buffer behavior is also important constituent while considering the process of diffusion of ions present in the soil towards the plant roots. The equilibrium between the soluble and solid bound ions gets reestablished when atleast a part of the solid phase ions get released into the solution. This soil buffer power can be explained by the following equation

b=?C?CL

Where C refers to the total amount of ion participating in diffusion inclusing the soluble ions and the ones getting released from the solid surfaces, and CL is the concentration of the ion present in bound state.

Therefore, the process of diffusion gets affected by various factors- 1) concentration of nutrient in question, 2) volume of soil filled with water, 3) pore size and geometry within the soil, 4) capacity of soil to retain ion (Khorassani 2008).

The uptake of nutrients by the plant roots get limited by the rate of transportation across the soil especially when the concentration of P is quite low. When the plant absorbs the P available near the root surface, a depletion zone gets created in the local region. The extent of depletion zone can be calculated as the following:

?x=Det

where x represents the extent of the depletion zone. It is the measure of the distance on the curve to the point where the concentration of P diffusing increases by 20% compared to the concentration around the root surface. The concentration gradient gets established towards the root as the amount of P surrounding the roots start depleting. As known the rate of diffusion depends on the gradient of concentration, it can be increased by either uptake of P at root surface thereby decreasing its amount in the surrounding areas or by increasing the concentration of free P in the soil (Khorassani 2008).

Conclusion

Phosphorus is an important part of many structural components found in plants, which mainly include nucleic acids and membranes. It is important as it helps in storing the energy produced during the process of photosynthesis. It also helps in regulating many important reactions within the plant cells by the process of phosphorylation and de-phosphorylation. There are many factors which govern the availability of phosphorus to plants. Only the phosphorus present as orthophosphate ions in soluble form can be easily taken up by the plants and the movement of these free ions occurs by the process of diffusion. The main factors influencing the availability of phosphorus can be summarized as chemical nature of phosphorus or the form in which it is present in the soil, the physical characteristics of the soil, the rate at which it is transported to the roots and the capacity of the plant to uptake phosphorus from the soil.

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